Interleukin-6 increases sensitivity to the locomotor-stimulating effects of amphetamine in rats

Interleukin (IL)-6 mediates brain-immune interactions, influences the survival of postnatal mesencephalic and basal forebrain cells, influences mesocorticolimbic dopamine and serotonin neurotransmission, and is linked with various central nervous system disorders. In the present study, single injections of IL-6 (1 or 2 microg/Long-Evans rat, i.p.) induced modest elevations of locomotor activity. The locomotor increases were not augmented by repeated intermittent injections of IL-6 (five daily injections; 1 microg/rat), however. Nonetheless, repeated IL-6 treatment increased sensitivity to the locomotor-stimulating effects of 1.0 and 0.5 mg/kg amphetamine, when tested 5, 7, or 14 days following interruption of the cytokine treatment. The ability of acute IL-6 injections to alter locomotor activity and the ability of repeated IL-6 injections to produce long-lasting sensitization to the locomotor-stimulating effects of amphetamine suggest an interaction of this cytokine with the mesolimbic dopamine system, a system implicated in aspects of schizophrenia, addiction, and movement disorders. The fact that IL-6 caused a lasting change in responsiveness to amphetamine implies a mechanism by which immunogenic stimuli can alter brain circuitry, changing its sensitivity to seemingly unrelated subsequent stimuli or events.